1. Technical Field of the Invention
The present invention relates in general to spectrometers, and in particular to Fourier Transform micro spectrometers.
2. Description of Related Art
Absorption spectrometers measure how well a sample absorbs light at each wavelength. Most absorption spectrometers utilize a “dispersive spectroscopy” technique, but others utilize a “Fourier transform spectroscopy” technique. The main difference between a Fourier transform (FT) spectrometer and other dispersive-type spectrometers (or spectrometers based on tunable wide-free-spectral-range high-finesse Fabry-Perot filters) is that an FT spectrometer measures all the wavelengths coincidentally, while other types of spectrometers measure one wavelength a time. As a result, FT spectrometers have higher measuring speeds and larger signal to noise ratios than dispersive spectrometers.
FT spectrometers are usually based on Michelson interferometers, in which collimated light from a broadband source is split into two beams, which are then reflected off of respective mirrors (one of which is moving) and caused to interfere, allowing the temporal coherence of the light to be measured at each different Optical Path Difference (OPD) offered by the moving mirror. The resulting signal, called the interferogram, is measured by a detector at many discrete positions of the moving mirror. The measured spectrum is retrieved using a Fourier transform carried out by a signal processor.
The interferogram of a single wavelength coherent source is periodic and varies with the OPD by a cosine function. Ideally, measuring any part of the interferogram would result in the same spectrum. Broadband sources, however, have most of the interferogram power concentrated around the zero OPD. Therefore, the moving mirror travel range should cover this portion of the interferogram. This is usually achieved by letting the respective distances between the beam splitter and each of the mirrors be equal (or close to it) and moving the mirror such that the distance between the beam splitter and the moving mirror assumes both negative and positive values with respect to the OPD position resulting in the detection of a double-sided interferogram. The maximum travel range scanned by the moving mirror (i.e. actuator travel range) governs the resolving power of an FT spectrometer. The larger the travel range, the better the resolution such that the resolution is inversely proportional to the travel range.
Many versions of the FT spectrometer based on Michelson interferometry have been developed based on the motion of an in-plane mirror or out-of-plane mirror with respect to the substrate. FT spectrometers based on Fabry-Perot (FP) interferometers, instead of Michelson interferometers, have also been developed. However, in FP-based FT spectrometers, the zero OPD can be achieved only by bringing the two optical surfaces in physical contact, which is impractical especially with actuation. Therefore, FP-based FT spectrometers are typically designed such that the partially reflective optical surfaces are left fixed in position but the gap between them has a varying function in space. The detector may then be composed of many small detectors (pixels) spread in space in order to capture the transverse interferogram (transverse with respect to the optical axis of the reflective optical surfaces) in a manner similar to imaging.
An FT spectrometer based on two FP interference transmission filters has also been developed in order to avoid the use of Michelson interferometry and it's mechanically moving parts. The two FP filter layer thicknesses may be set to the same value, and the optical layer thickness of one of them may be modulated by means of electro-optical, accousto-optical, thermo-optical or piezo-electrical methods without using moving parts. The use of the envelope of the transmission curve, resulting from the superposition of the airy functions of the two FP interference filters, enables the detection of the radiation flux starting from zero OPD and larger.
FT spectrometers based on Mach-Zehnder (MZ) interferometers can only achieve zero OPD between the two optical paths (OP1 and OP2) if different substances are used in the two paths. For example, a silicon half-plane beam splitter can be used such that the optical beam OP1 is in silicon and the other optical beam OP2 is in another substance. In this case, zero OPD can be achieved, but with restrictions on the size of the device. What is needed is a spectrometer with better compactness and improved resolution that can be monolithically integrated and is able to capture double-sided interferograms.